Using a Hydrometer

ORIGINAL GRAVITY

After the full batch is in the fermenter, stir, shake, slosh - whatever - to
ensure it is thoroughly mixed, then transfer some to your hydrometer flask.

I
use a sanitized turkey baster to slurp some from the fermenter, then I squirt
it into the hydrometer flask with the hydrometer already in it. That way I
get just enough. Read the hydrometer and write the date and the reading down. The diagram to the right demonstrated how to read the gravity using the hydrometer. Note that you read the lowest portion og the miniscus.

Then, take the hydrometer out and put it aside and put a thermometer in the
sample. Read the temperature and write this in your log as well.

NEVER, EVER, EVER return a sample to the fermenter unless your turkey baster (or whatever means you use), flask, hydrometer and thermometer have been THOROUGHLY sanitized! I usually just drink mine...

RACKING GRAVITY

If you rack to a secondary, take a sample from the siphon - again, with the
hydrometer in the flask so you get just enough. Tap the flask and spin the
hydrometer to free any CO2 bubbles, then let it sit while you finish
siphoning to your secondary (Should take 15 minute to a half hour). Tap and
spin again, then read the hydrometer. Write this reading in your log along
with the date. Read the sample's temperature, and add this to your log as
well.

FINAL GRAVITY

While siphoning to your keg or bottling bucket, do the same as for racking
gravity. This reading should be in the neighborhood of 1/3 to 1/4 of your
original gravity; 1/4 being the most commonly cited number.

Having this data not only gives you a second reference to completed
fermentation, but it will also help you in sharing and duplicating your
recipes.

Offsets and such...

Lou Heavner points out:

...One thing you might want
to add is a paragraph on how to initially calibrate/validate
a hydrometer. As a reader of HBD for a couple of years, it
seems like a common possibility for unusual SG's is the
hydrometer scale may be slighty misplaced. I know mine was
off by a couple points. It's easy to try it in distilled H2O
at 60 DegF to establish confidence. Just a thought....

And a very good thought it is, too! I "calibrate" mine by testing it at temperature increments from about 45°F to 170°F, then I plot the result and use multiple regression to fit a model to the data. This method helps me to adjust the reading of my hydrometer within what I consider to be its usable range (I also use the hydrometer to test for the end of sparging). But I'm a statistical geek, too.

To test your hydrometer as Lou suggests, fill your hydrometer flask with distilled water at 60°F (most hydrometers are calibrated to be accurate at this temperature). Float your hydrometer in it and tick it and spin it to free any clinging bubbles. Read the value on the hydrometer, record it, then check the temperature again (to ensure it is still at 60°F). Do this a few times and average the readings. This is your "offset" - the amount above or below the actual gravity that your hydrometer is reading.

To take it further, you can determine your own correction factor by heating the water sample by five or ten degrees at a time, taking gravity measurements, and then fitting a curve to your data.